Ribosome Assembly

1. tRNAs are transcribed in the nucleoplasm by RNA polymerase III. The primary transcript is processed to the mature tRNA and transported to the cytoplasm. In the cytoplasm the tRNAs are coupled to their cognate amino acids.
amino acid activation»
tRNAprocessing»

2. Pre-mRNAs, containing the sequences coding for the ribosomal proteins, are transcribed by RNA polymerase II in the nucleoplasm. The pre-mRNAs are processed (see splicing) and transported (see transport) as mRNP-particles to the cytoplasm where they are picked up by the ribosomes
initiation and translated elongation into ribosomal proteins. The mature ribosomal proteins are then transported (see transport) to the nucleolus.

3. The ribosomal RNAs (except 5S rRNA) are transcribed in the nucleolus as one giant precursor RNA by RNA polymerase I. The precursor is processed (see rRNA processing) to 18S,5.8S and 28S rRNA. 5S rRNA is transcribed in the nucleoplasm by RNA polymerase III and transported to the nucleolus. The rRNAs are folded and associate with ribosomal proteins to form the 40S and 60S ribosomal subunits. The subunits are then transported from the nucleolus to the cytoplasm.

An example of the Rate of Ribosome Synthesis:

*HeLa cells (a type of human tumour cells) divide each 24 hours.
* Each cell contains around 10 million ribosomes, i.e. 7000 ribosomes are produced in the nucleolus each minute.
*Each ribosome contains around 80 proteins, i.e. more than 0.5 million ribosomal proteins are synthesised in the cytoplasm per minute.
*The nuclear membrane contains approximately 5000 pores. Thus, more than 100 ribosomal proteins are imported from the cytoplasm to the nucleus per pore and minute. At the same time 3 ribosomal subunits are exported from the nucleus to the cytoplasm per pore and minute.